R F Hill Amateur Radio Club Presentation Part I: Dipoles By Joe Rauchut N3CRP January 31, 2007

Antenna Basics An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired.

Terms to Remember Impedance Matching For efficient transfer of energy, the impedance of the radio, the antenna, and the transmission line connecting the radio to the antenna must be the same. VSWR and Reflected Power The Voltage Standing Wave Ratio (VSWR) is an indication of how good the impedance match is. VSWR is often abbreviated as SWR. A high VSWR is an indication that the signal is reflected prior to being radiated by the antenna. Bandwith Bandwidth can be defined in terms of radiation patterns or VSWR/reflected power. If bandwidth is expressed in absolute units of frequency, for example MHz, the bandwidth is then different depending upon whether the frequencies in question are near 150, 450, or 825 MHz.

Terms to Remember Directivity The ability of an antenna to focus energy in a particular direction when transmitting or to receive energy better from a particular direction when receiving. Wavelength We often refer to antenna size relative to wavelength. For example: a half-wave dipole, which is approximately a half-wavelength long. Wavelength is the distance a radio wave will travel during one cycle. Db A most convenient way to compare power is as a ratio between two powers. For this purpose, we have the decibel or dB. One dB = 10 log (P1/P2), where P1 and P2 are any two powers it is relevant to compare. A power ratio of 2 is 3 dB; a ratio of 4 is 6 dB; a ratio of 10 is 10 dB. These marker points will guide you to intermediate values. Your transceiver output is 100 watts. Your linear output is 800 watts. Hence, your power gain is 9 dB to the antenna. Example: dB=10log (800/100) = 10log(8) = 9.03

Dipole Fundamentals A dipole is antenna composed of a single radiating element split into two sections, not necessarily of equal length. The RF power is fed into the split. The radiators do not have to be straight.

Dipole Characteristics Electrical length - the overall length of the dipole in wavelengths at the frequency of interest. Wavelength = 300/frequency in MHz Example 300/7.5 MHz = 40 Meters Self Impedance - the impedance at the antenna’s feed point (not the feed point in the shack). Radiation Resistance - a fictitious resistance that represents power flowing out of the antenna Radiation Pattern - the intensity of the radiated RF as a function of direction.

The Short Dipole The length is less than /2 The self impedance is generally capacitive. The radiation resistance is quite small and ohmic losses are high SWR bandwidth is quite small, ~ 2% of design frequency. Directivity is ~1.8 dBi. Radiation pattern resembles the figure to the right.

The Short Dipole For dipoles longer than /5, the antenna can be matched to coax by using loading coils For best results, the coils are placed in the middle of each leg of the dipole Loading coils can introduce additional loss of 1 dB or more For dipoles longer than /3 the antenna can be matched to coax by using linear loading

Design Table: Short Dipole Design Height: 60 ft. Feed point impedance: 40  /4 dipole with inductive loading 0.36 dipole with linear loading

The Half Wave ( /2) Dipole Length is approximately /2 (0.48 for wire dipoles) Self impedance is ohms with no reactive component (good match to coax) Directivity ~ 2.1 dBi SWR Bandwidth is ~ 5% of design frequency Example L=468/7.25MHz=64.55 feet

Harmonic Operation of /2 Dipoles A /2 dipole is also resonant at integral multiples of its resonant frequency - harmonic. The self impedance of a /2 dipole at odd multiples of the resonant frequency is ohms. The self impedance at even multiples is > 1000 ohms The directivity is never greater than the extended double Zepp. The pattern is very complex, with many side lobes.

Design Table: Half Wave Dipole

The Full Wave Dipole (Double Zepp) Length is approximately (0.99 for wire dipoles) Self impedance is ~ 6000 ohms. Antenna can be matched to coax with a 450 ohm series matching section Directivity ~ 3.8 dBi SWR Bandwidth ~ 5% of design frequency Zepp Antenna: An end fed antenna used on the big zeppelin airships (thus the name ZEPPELIN antenna) of the 1930's where the wire trailed behind the dirigible.

17 Meter double Zepp Antenna

Design Table: Double Zepp

The Extended Double Zepp Length is approximately 1.28 Self impedance is approx j800 ohms Antenna can be matched to 50 ohm coax with a series matching section Directivity ~ 5.0 dBi. This is the maximum broadside directivity for a center-fed wire antenna

Design Table: Extended Double Zepp

The 3 /2 Dipole Length is approximately 1.48 Self impedance ~ 110 ohms Antenna can be matched to 50 ohm coax with quarter wave 75 ohm matching section Directivity ~ 3.3 dBi. Directions of max radiation point to all areas of interest for HF DX when antenna wire runs E-W

Design Table: 3 /2 Dipole

Dual Band Dipole It is possible to select the length of a dipole and its series matching section such that low SWR can be obtained on two bands The SWR bandwidth of this type of dipole is less than a regular dipole; full band coverage is not possible on most HF bands Note: the dipole alone is generally not resonant on either band

Design Table: Dual Band Dipole

Off-Center Fed Dipole (OCD) By moving the feed point away from the center, it is possible to have a low feed point impedance at frequencies other than the odd multiples of the resonant frequency The feed point impedance of an OCD is > 100 ohms, necessitating use of a transformer at the feed point The relationship between feed position and feed impedance is very complex, but in general as the feed moves away from the center, the impedance increases and the number of harmonics with low impedance resonance increases.

Design Table: OCD antennas

Use of a dipole on several bands It is possible to use a center fed dipole over a wide range of frequencies by: –feeding it with low-loss transmission line (ladder line) –providing impedance matching at the transceiver The lower frequency limit is set by the capability of the matching network. Typically a dipole can be used down to 1/2 of its resonant frequency. The radiation pattern becomes very complex at higher frequencies. Most of the radiation is in two conical regions centered on each wire There is no special length, since the antenna will not be resonant

The G5RV: what is it, really? The G5RV was originally designed as a 3 /2 antenna for use on 20 meters. It was used as a multi-band antenna because when fed with ladder line (not coax!) it is easy to match the on any band from 80m to 10m A G5RV used as a multi-band antenna should be fed with ladder line. Most commercially-made G5RV antennas are lossy because they are fed with coax.

Dipole Polarization On the HF bands dipoles are almost always horizontally polarized. It is not possible to get a low angle of radiation with a vertical dipole (electrically) close to the earth Reflection losses are also greater for vertically polarized RF The height of the support required for a vertical dipole can also be a problem

Putting up a Dipole A dipole may be erected between 2 supports or with one support. A dipole antenna using a single support is known as an “inverted-V” The legs of a dipole may also be bent to form an inverted U. The bend should be at least half way to the end of the wire

Dipole Antenna Materials Wire –#14 Copperweld very strong kinks very easily; it is difficult to work with does not stretch subject to corrosion –#14 stranded copper wire with vinyl insulation moderately strong easy to work with, does not kink can stretch under high tension (a problem with long antennas) does not corrode –Monel trolling wire strong much higher resitivity than copper corrosion resistant

Dipole Antenna Materials Insulators Ceramic strong resist very high voltages not affected by sunlight expensive Plastic weaker than ceramic insulators resist moderately high voltages can be degraded by sunlight relatively inexpensive

Dipole Antenna Materials Baluns –choke balun (several turns of coax wound into coil ~ 6 in in dia) is usually sufficient unless impedance transformation is required –Powdered-iron core baluns should be used within their ratings to avoid core saturation. Support ropes –should be at least 3/16 inch diameter and UV stabilized –UV stabilized Dacron works well in most applications –polyolefin ropes quickly degrade in sunlight and should be avoided

Dipole Antenna Supports Almost any structure can be used to support a dipole The antenna should be kept at least 12 inches away from a conducting support. If trees are used, leave some slack in the antenna so that swaying of the branches does not snap the wire The support should be tall enough that the dipole is at least 1/2 wavelength about the surrounding terrain ( /2 =492/f)

Other useful information Do not run a dipole above power lines!!!! When the feed line leaves the dipole, it should run perpendicular to the dipole for at least 1/4 wavelength Avoid running the dipole parallel to long conducting objects such as aluminum gutters. The antenna can couple to the other metal and be detuned When erecting a dipole as an inverted-V, remember that the voltage at the ends of the antenna may be above 1000 V. The ends of the antenna should not be so close to ground that a person could touch them When erecting an inverted-V, the angle between the wires should be greater than 90 degrees

Antenna Comparison

73 _ _ _ _ This PowerPoint Program is on the R F Hill Website